Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic image forming system.
Description of the Related Art
Conventionally, an image forming step performed in an image forming apparatus employing an electrophotographic system includes evenly charging the surface of a drum-type electrophotographic photosensitive body (hereinafter called a photosensitive drum) at a prescribed potential. For charging, a contact charging system has been recently mainstream in which a roller charging member (hereinafter called a charging roller) is, for example, brought into contact with the surface of a photosensitive drum and a voltage is applied to the charging roller to charge the photosensitive drum. As methods for applying a voltage to a charging roller, there have been known a method for applying a direct current voltage and a method for superimposing an alternating current voltage on a direct current voltage and alternately generating a discharge on positive and negative sides to uniformize a charge. In the latter method, a resistive load current flowing to the resistive load between a charging roller and a photosensitive drum, a capacitive load current flowing to the capacitive load between the charging roller and the photosensitive drum, and a discharge current between the charging roller and the photosensitive drum flow, and a current in which these currents are added together flows to the charging roller. In this case, it is known from experience that setting a discharge current amount at a prescribed value or more makes it possible to obtain a stable charge.
FIG. 18 shows the characteristics of a current Ic that flows to a charging roller when an alternating current voltage Vc is applied to the charging roller. The alternating current voltage Vc indicates the peak voltage value of an alternating current voltage, and the current Ic indicates the effective value of an alternating current. It can be seen from FIG. 18 that together with a gradual increase in the amplitude of the alternating current voltage Vc, a charging current increases concomitantly. When the alternating current voltage Vc is twice or less a prescribed voltage Vh, the amplitude of the alternating current voltage and the charging current are approximately proportional to each other. This is because a resistive load current and a capacitive load current are proportional to the amplitude of the voltage and also because a discharge phenomenon does not occur since the amplitude of the voltage is small and so a discharge current does not flow. Then, as the alternating current voltage is further increased, the discharge phenomenon starts to occur at a point where the prescribed voltage Vh is doubled. At this point, the charging current Ic starts to deviate from being proportional to the alternating current voltage Vc, and its flow increases by an amount corresponding to a discharge current Is. Here, in order to obtain a stable charge, it is necessary to set the alternating current voltage Vc such that the discharge current Is is a prescribed value or more.
However, when the amount of a discharge from a charging roller to a photosensitive drum increases, there are cases that the degradation of the photosensitive drum such as scraping of the photosensitive drum is accelerated and an abnormal image based on image deletion, etc. under high temperature and high humidity due to a discharge product occurs. Accordingly, in order to solve the above problems, it is necessary to obtain a stable charge and control the application of a minimum voltage so as to suppress a discharge amount as much as possible. In reality, however, the relationship between a voltage applied to a photosensitive drum and a discharge amount is not always constant and changes depending on the film thickness of the photosensitive layer or the dielectric layer of the photosensitive drum, the environmental variation of a charging member or air, or the like. It has been known that problems that accompany a change in a discharge amount are associated not only with a reason including the above environmental variation but also with a resistance value variation due to the manufacturing fluctuation or stain of a charging member, the electrostatic capacitance variation of a photosensitive drum due to durability, the characteristics fluctuation of the high pressure generation device of an image forming apparatus body, or the like.
In order to suppress such a change in a discharge amount, “discharge current control methods” such as that indicated in Japanese Patent Application Laid-open No. 2004-157501 have been proposed. In addition, as described above, electric characteristics of a photosensitive body change depending on the use environment or the manner in which it is used. Particularly, it has been known that a change in film thickness results in a change in image density and affects the density or the like of an output image, which causes a change in an image. Japanese Patent Application Laid-open Nos. 2011-118234 and 2012-13881 have each proposed a method for detecting a discharge start voltage according to the Paschen's law to easily detect the potential state of a photosensitive body as a method for detecting the potential state of the surface of a photosensitive drum.
However, along with the long service life of an image forming apparatus or the diversification of a using method in the market that has been developed in recent years, there has been a case that unevenness occurs in the stain of a charging roller or a change in the film thickness of a photosensitive drum in a longitudinal direction. The problem is likely to occur, for example, when a biased image is continuously formed by a printing unit in a direction (hereinafter called a longitudinal direction) perpendicular to an image formation processing direction (the conveyance direction of a recording material). Besides, the problem is likely to occur when a small recording material such as an envelope and a postcard is continuously used in an image forming apparatus having a system in which a photosensitive drum and a recording material such as a sheet directly come in contact with each other.
After the above biased printing or the feeding of small recording materials is performed for a long period of time, the occurrence of unevenness in the longitudinal direction of the film thickness of a photosensitive drum results in different impedance in the longitudinal direction, whereby a difficulty in discharging is made different for each part in the longitudinal direction. According to the above discharge current control method, a discharge amount in an entire longitudinal direction is detected. Therefore, there are a part where a discharge amount is smaller than an appropriate amount or a part where a discharge amount is greater than the appropriate amount and a scraping amount of a photosensitive drum is increased. When the unevenness of film thickness in a longitudinal direction occurs with an increase in the scraping amount of a photosensitive drum, the potential state of a laser exposure unit is made different. The potential difference of the exposure unit results in a difference in development contrast, and thus development performance is made different in the longitudinal direction. Accordingly, when an appropriate potential for image formation is not evenly obtained in the longitudinal region of the photosensitive drum, an image failure such as density unevenness occurs in the longitudinal direction.
It is preferable to foresee the unevenness of the film thickness of a photosensitive drum in advance to determine a discharge amount, but there is a case that the foreseeing is difficult since such a phenomenon greatly changes depending on the use conditions of a user.